1. Field of the Invention
This invention relates to an image forming apparatus such as a laser beam printer or a copying apparatus, and in particular to a fixing device for fixing unfixed images by applying heat thereto.
2. Related Background Art
In the latest image forming apparatuses, recording on recording sheets of various sizes such as, for example, formats A3, B4, A4 and B5, is possible. Also, as a device for fixing unfixed toner images formed on recording sheets, it is widely known to apply heat and/or pressure to the recording sheets by a heat plate, a heat roller or the like to thereby melt toner. Particularly, from the requirements for compactness and low power consumption of the apparatus, heat roller fixing devices have come into wide use.
For example, in a heat roller fixing device, the roller length must be the maximum recording paper size, e.g. A3 size or larger. When a recording sheet of small size such as A4 or B5 size is passed between the fixing rollers, as shown in FIG. 3, there is created in the maximum sheet passage width l.sub.L of the fixing rollers a portion through which the recording sheet does not pass (hereinafter referred to as the "sheet non-passage portion")-the portion l.sub.L-S in FIG. 3.
When recording sheets of small size are continuously passed, heat is not taken from the roller surface in the sheet non-passage portion l.sub.L-S by the recording sheets and therefore, the temperature of the roller surface in the sheet non-passage portion l.sub.L-S becomes very high and the temperature difference between the sheet passage portion l.sub.S and the sheet non-passage portion l.sub.L-S sometimes becomes 60 deg. or higher.
In the case as described above, if recording sheets of small size are continuously passed, the temperature of the roller surface in the sheet non-passage portion l.sub.L-S becomes very high. This has led to the following disadvantages:
(1) When a recording sheet of large size is fixing immediately thereafter, high temperature offset is caused in that portion of the sheet which corresponds to the sheet non-passage portion l.sub.L-S and the quality of image is deteriorated; PA0 (2) The diameter of the fixing roller in the sheet non-passage portion l.sub.L-S is thermally expanded and the diameter of the fixing roller becomes axially non-uniform to cause the recording sheet to be wrinkled; PA0 (3) The thermal deterioration of the offset preventing layer on the surface of the fixing roller is expedited to thereby shorten the service life of the fixing roller; and PA0 (4) A separating pole which is in contact with the fixing roller to prevent the recording sheet from twining on the fixing roller is deformed by heat.
Now, in order to prevent such conveniences, there is known the technique of making the heat distribution of a heater for heating the fixing roller such that the amount of heat supplied to the sheet non-passage portion l.sub.L-S is smaller than the amount of heat supplied to the sheet passage portion l.sub.S for a recording sheet of small size. However, when a recording sheet of large size is fixated, unsatisfactory fixing such as low temperature offset becomes liable to occur in the portion corresponding to the sheet non-passage portion l.sub.L-S because the amount of heat supplied to the sheet non-passage portion l.sub.L-S is small.
According to the result of the inventor's experiment, it is considered to be difficult to satisfy the fixativeness of a recording sheet of large size and to prevent the temperature rise of the sheet non-passage portion during continuous passage of recording sheets of small size simply by contriving the heat distribution of the heater.